Porous composite membrane and method for making the same

ABSTRACT

The invention provides composite porous membranes comprising a porous hydrophobic substrate coated with difunctional surface-modifying molecules. The difunctional surface-modifying molecules provide a hydrophilic surface without forming branches of interconnected polymer molecules in the pores. The invention also provides a method for making composite porous membranes, such as a composite hydrophilic membrane with reduced concentration of surface modifying molecules required to coat a hydrophobic substrate.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority of U.S. Provisional Application60/407,856, filed Sep. 3, 2002, the disclosure of which is incorporatedby reference herein.

FIELD OF THE INVENTION

The invention relates to a porous composite membrane with a hydrophilicsurface and a method for making the same.

BACKGROUND OF THE INVENTION

Many synthetic polymeric membranes are made from hydrophobic polymersbecause they have desirable bulk properties such as flexibility, thermalstability, and chemical stability. However, the surfaces of suchmembranes are not suitable for applications requiring interactions withaqueous solutions, low protein adsorption, controlled ion exchangecapacity, and controlled surface chemical reactivity.

It is often desirable to provide a porous membrane with a hydrophilicsurface, which nevertheless retains the bulk properties of theunderlying hydrophobic membrane. Such membranes are important infiltration applications that require the passage of aqueous fluidsthrough the membranes. Additionally, porous hydrophilic membranes haveimportant biological applications (e.g., as implantable medicaldevices), and use in assays relying on the capture and/or immobilizationof biomolecules (e.g., nucleic acids or proteins) on a membrane surface.Therefore, the process of coating a hydrophobic surface should notdiminish flow through efficiency of the membrane. Thus, processes thatminimize pore clogging are essential for generating useful membranescomprising hydrophilic surfaces.

To render hydrophobic membranes hydrophilic, a wetting agent, such as asurface-active agent, can be added to a polymeric system being used tocast the membrane. Typically such coatings are only temporary, and themembrane so coated cannot be subjected to repeated wetting and dryingprocedures without loss of wettability. Further, exposure to any processfluid can generally extract the coating. This is particularlyundesirable when processing biological fluids or contacting cells whosecontinued viability is desired.

Additional methods of casting membranes rely on the inclusion ofhydrophilic cross-linkable monomers in a casting solution of dissolvedhydrophobic polymer. Upon casting, a semi-crystalline polymer withhydrophilic surface properties is formed. See, e.g., U.S. Pat. Nos.5,079,272 and 5,158,721.

Another method of preparing hydrophilic membranes involves graftpolymerizing a hydrophilic monomer onto the surface of a poroushydrophobic polymeric membrane substrate. A typical example of aphotochemical grafting process used to modify a hydrophobic surface withhydrophilic polymers is described in U.S. Pat. No. 5,468,390.

A number of patents also describe the covalent immobilization ofhydrophilic polymers to a hydrophobic substrate using a photoreactivemolecule covalently bound to the polymer, i.e., through a linkingmolecule. See, e.g., U.S. Pat. Nos. 4,973,493; 4,979,959; 5,002,582;5,217,492; 5,258,041; 5,263,992; 5,414,075; 5,512,329; 5,563,056;5,637,460; and 5,714,360.

U.S. Pat. No. 4,917,793 discloses directly coating a cross-linkedpolymer having desired surface properties on porouspolytetrafluoroethylene membrane. The polytetrafluoroethylene membraneis exposed to a reagent bath comprising a free radical polymerizablemonomer, a polymerization initiator and cross-linking agent (e.g., suchas a difunctional molecule) in a solvent comprising water and a watermiscible, polar, organic solvent under conditions to effect free radicalpolymerization of the monomer and coating of the porous membrane withthe cross-linked polymer. The use of chemical crosslinking reagents thatare typically tetrafunctional, results in highly branchedthree-dimensional structures that reduce the membrane's flow-throughefficiency by plugging pores. Generally, rapid pore blockage isassociated with the formation of an interpenetrating network ofcross-linked hydrophilic difunctional molecules in high concentrations(see, e.g., as shown in FIG. 1A).

Such a method of modifying hydrophobic surfaces with hydrophilicmolecules generally has the disadvantage of trapping excessive polymeron the membrane. This phenomenon can rapidly plug membrane poresirreversibly, leading to a rapid decline in flow rate and an increase inpressure required to filter molecules through the membrane. Further,membranes produced have high levels of extactables and demand longerrinsing cycles. Additionally, processes for making such membranes mayrequire significant amounts of coating monomer or polymer (e.g., 6-12%)and a long incubation time to achieve a uniformly coated surface.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a composite porous membranecomprising a hydrophobic substrate (e.g., such as polyvinylidenefluoride or PVDF) stably associated with crosslinked network ofdifunctional surface modifying molecules. The difunctionalsurface-modifying molecules comprise a hydrophobic portion and ahydrophilic portion and are preferentially associated with substrate viatheir hydrophobic portions. Preferably, greater than about 10%, greaterthan about 20%, greater than about 30%, greater than about 40%, greaterthan about 50%, greater than about 60%, greater than about 70%, greaterthan about 80%, greater than about 90%, up to about 100% of themolecules associated with the substrate comprise the difunctionalsurface-modifying molecules. More preferably, all of the moleculesassociated with the substrate comprise difunctional surface-modifyingmolecules.

Suitable hydrophobic groups include, but are not limited to, hydrophobicalkyl, aromatic group, or olefinic hydrocarbon groups. In one preferredaspect, the aromatic group comprises a bisphenol A group. Preferably,the aromatic group does not form covalent bonds with the substrate.

Preferably, the hydrophilic portion of the surface-modifying moleculescomprises at least two crosslinking active groups. More preferably, onegroup facilitates polymerization of the molecules, while the other groupfacilitates crosslinking between polymerized molecules. In one aspect, acrosslinking active group comprises a carbon-carbon double bond oranother chemical group capable of free radical formation after hydrogenabstraction. Suitable hydrophilic groups comprise the general formula[—X_(n1)—Y—CR═CH₂]_(n2) where X is independently selected from the groupincluding, but not limited to, X═(—CH2-CH2-O—);(—CH2-O—);(—CH2-CH(COOH)—); (—CH2-CH(OH)—), Y can include, but is notlimited to ([—CH2-]_(n3)); (—COO—); and n₁ is from about 1-50 while n₂is from about 1-2. n₃ can be from about 1 to about 50.

Preferably, difunctional surface modifying molecules are polymerized onthe substrate surface after preferentially absorbing to the substratesurface via the hydrophobic portions of the molecules.

In one aspect, difunctional surface-modifying molecules comprisedifunctional acrylate molecules. In one particularly preferred aspect,the difunctional surface-modifying molecules comprise ethoxylated (30)bisphenol A diacrylates.

The preferred free radical initiator for the present invention consistsof substantially hydrophobic (“phobic”) molecule, capable ofphobic-phobic interaction with a phobic surface of the substrate,resulting in the preferential adsorption of the photoinitiator moleculeon the substrate surface prior to the exposure to the UV-irradiation.

Composite membranes according to the invention have an average pore sizeof from about 0.01 μm to 10 μm, i.e., are suitable for microfiltration.Composite membranes also can be designed to be suitable forultrafiltration. Preferably, such membranes have molecular weight cutoffvalues of 10 kDa or less, 30 kDa, 50 kDa, 100 kDa, or higher and poresizes less than 0.1 μm.

In one aspect, the membrane is wettable within less than about 30seconds after drying upon contacting with an aqueous solution.Preferably, the membrane wets instantly after drying. Still morepreferably, the membrane can withstand repeated cycles of wetting anddrying. In a further aspect, the membrane is autoclavable.

The invention also provides a method for making a composite porousmembrane with a hydrophilic surface. The method comprises providing ahydrophobic substrate and coating the hydrophobic substrate withdifunctional surface modifying monomer molecules as described above, inthe presence of a photoinitiator and a solvent. In one preferred aspect,coating is performed using a flow-through method. The reagent solutioncomprising difunctional surface-modifying monomer is forced through thehydrophobic substrate using a driving force (e.g., such as a pressuredifferential, centrifugal force, and the like), maximizing the amount ofsurface-modifying molecules deposited and preferentially absorbed on thesubstrate.

BRIEF DESCRIPTION OF THE FIGURES

The objects and features of the invention can be better understood withreference to the following detailed description and accompanyingdrawings.

FIG. 1A is a schematic diagram showing the formation of a membrane ofthe prior art (i.e., without preferential adsorption). A poroushydrophobic substrate, indicated as heavy lined inter-crossing fibers inthe Figure, is exposed to a solution comprising surface-modifyingmolecules (shown as thin lined ellipses). Random, non-preferentialassociation of the surface-modifying molecules with the substrateoccurs, and the molecules are as likely to remain in solution as tobecome affixed to the substrate. Exposure to ultraviolet (UV) lightcauses the surface-modifying molecules to polymerize (illustrated by theend-to-end association between the surface-modifying molecules). Inaddition, crosslinking between the polymers causes the rapid formationof an interpenetrating network of cross-linked molecules. As can be seenfrom the Figure, this leads to rapid plugging of pores in the substrate(illustrated as open spaces between the fibers).

FIG. 1B shows formation of a composite membrane according to one aspectof the invention (i.e., with preferential adsorption). A poroushydrophobic substrate is exposed to difunctional surface moleculescomprising a hydrophobic portion and a hydrophilic portion. Thesemolecules preferentially absorb to the substrate via the hydrophobicportions of the molecules and therefore are more likely to be retainedon the substrate than the surface-modifying molecules shown in FIG. 1A.Polymerization upon exposure to UV light results in moresurface-modifying molecules on the substrate surface, and thereforecrosslinking results in few, if any, polymer chains plugging the poresof the substrate. Providing hydrophobic photoinitiator molecules tofacilitate the polymerization and cross-linking process enhances thiseffect. Such molecules are preferentially deposited on the substrate andtherefore initiate polymerization on the substrate rather than acrosspores as shown in FIG. 1A.

FIGS. 2A-2O show exemplary hydrophobic photoinitiator molecules that canbe used in methods of the invention. For mixtures, percentages are givenby weight.

FIG. 2A shows the chemical structure of1-hydroxy-cyclohexyl-phenyl-ketone (molecular weight 204.3; CibaSpecialty Chemicals; Ciba® IRGACURE® 184; CAS No. 947-19-3).

FIG. 2B shows the chemical structure of2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (molecularweight 366.5; Ciba Specialty Chemicals; Ciba® IRGACURE® 369; CAS No.119313-12-1).

FIG. 2C shows the chemical structures of1-hydroxy-cyclohexyl-phenyl-ketone (top; see also FIG. 2A) andbenzophenone (bottom; see also FIG. 2J), which may be used as a mixture(e.g., 50% 1-hydroxy-cyclohexyl-phenyl-ketone and 50% benzophenone(w/w); Ciba Specialty Chemicals; Ciba® IRGACURE® 500).

FIG. 2D shows the chemical structures ofbis(2,6-dimethoxybenzoyl)-2,4,4-trimethyl pentyl phosphineoxide (top)and 2-hydroxy-2-methyl-1-phenyl-propan-1-one (bottom; see also FIG. 2I),which may be used as a mixture (e.g., 25%bis(2,6-dimethoxybenzoyl)-2,4,4-trimethyl pentyl phosphineoxide and 75%2-hydroxy-2-methyl-1-phenyl-propan-1-one (w/w); Ciba SpecialtyChemicals; Ciba® IRGACURE® 1700).

FIG. 2E shows the chemical structure of2,2-dimethoxy-1,2-diphenylethan-1-one (molecular weight 256.3; CibaSpecialty Chemicals; Ciba® IRGACURE® 651; CAS No. 24650-42-8).

FIG. 2F shows the chemical structure ofbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (molecular weight418.5; Ciba Specialty Chemicals; Ciba® IRGACURE® 819; CAS No.162881-26-7).

FIG. 2G shows the chemical structures of2-hydroxy-2-methyl-1-phenyl-propan-1-one (top; see also FIG. 2I) and1-hydroxy-cyclohexyl-phenyl-ketone (bottom; see also FIG. 2A), which maybe used as a mixture (e.g., 80% 2-hydroxy-2-methyl-1-phenyl-propan-1-oneand 20% 1-hydroxy-cyclohexyl-phenyl-ketone (w/w); Ciba SpecialtyChemicals; Ciba® IRGACURE® 1000).

FIG. 2H shows the chemical structures ofbis(2,6-dimethoxybenzoyl)-2,4,4-trimethyl-pentylphosphineoxide (top) and1-hydroxy-cyclohexyl-phenyl-ketone (bottom; see also FIG. 2A), which maybe used as a mixture (e.g., 25%bis(2,6-dimethoxybenzoyl)-2,4,4-trimethyl-pentylphosphineoxide and 75%1-hydroxy-cyclohexyl-phenyl-ketone (w/w); Ciba Specialty Chemicals;Ciba® IRGACURE® 1800).

FIG. 2I shows the chemical structure of2-hydroxy-2-methyl-1-phenyl-propan-1-one (molecular weight 164.2; CibaSpecialty Chemicals; Ciba® DAROCUR® 1173; CAS No. 7473-98-5).

FIG. 2J shows the chemical structure of benzophenone (molecular weight182.2; Ciba Specialty Chemicals; Ciba® DAROCUR® BP; CAS No. 119-61-9).

FIG. 2K shows the chemical structures of2,4,6-trimethylbenzoyl-diphenyl-phosphineoxide (top) and2-hydroxy-2-methyl-1-phenyl-propan-1-one (bottom; see also FIG. 2I),which may be used as a mixture (e.g., 50%2,4,6-trimethylbenzoyl-diphenyl-phosphineoxide and 50%2-hydroxy-2-methyl-1-phenyl-propan-1-one (w/w); Ciba SpecialtyChemicals; Ciba® DAROCUR® 4265; CAS Nos. 75980-60-8 and 7473-98-5).

FIG. 2L shows the chemical structure ofbis(η-5-2,4-cyclopentadien-1-yl)-bis(2,6-difluoro-3-(1H-pyrrol-1-yl)-phenyl)titanium (molecular weight 534.4; Ciba Specialty Chemicals; Ciba®IRGACURE® 784; CAS No. 125051-32-3).

FIG. 2M shows the chemical structure of2-methyl-1[4-(methylthio)phenyl]-2-morpholinopropan-1-one (molecularweight 279.4; Ciba Specialty Chemicals; Ciba® IRGACURE® 907; CAS No.71868-10-5).

FIG. 2N shows the chemical structures of2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (top; seealso FIG. 2B) and 2,2-dimethoxy-1,2-diphenylethan-1-one (bottom; seealso FIG. 2E), which may be used as a mixture (e.g., 30%2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 and 70%2,2-dimethoxy-1,2-diphenylethan-1-one (w/w); Ciba Specialty Chemicals;Ciba® IRGACURE® 1300; CAS Nos. 119313-12-1 and 24650-42-8).

FIG. 2O shows the chemical structure of1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one(molecular weight 224.3; Ciba Specialty Chemicals; Ciba® IRGACURE® 2959;CAS No. 106797-53-9).

DETAILED DESCRIPTION

In one aspect, the invention provides composite porous membranescomprising a porous hydrophobic substrate coated with difunctionalsurface-modifying molecules. The difunctional surface-modifyingmolecules provide a hydrophilic surface without affecting the desirablebulk properties of the underlying substrate. The invention also providesa method for making composite porous membranes, such as a compositehydrophilic membrane with reduced concentration of surface-modifyingmolecules required to coat a substrate to the levels not practicedbefore in the art.

Definitions

The following definitions are provided for specific terms which are usedin the following written description.

As used in the specification and claims, the singular form “a”, “an” and“the” include plural references unless the context clearly dictatesotherwise. For example, the term “a molecule” also includes a pluralityof molecules.

The term “pore size” refers to the minimum size of particles that willbe retained on the membrane. Thus, a membrane with a pore size of about0.45 microns means that particles greater than about 0.45 microns willbe retained on the membrane, those less than about 0.45 microns willpass through and will not be retained.

As used herein, “a difunctional surface-modifying molecule” refers to amolecule which comprises a hydrophobic portion and a hydrophilic portionand at least two crosslinking active groups.

As used herein, a “crosslinking active group” refers to a chemical group(e.g., such as a carbon-carbon double bond) which is capable ofundergoing free radical polymerization.

As used herein, “preferential absorption” with respect to a difunctionalsurface-modifying molecule refers to the preference of the molecule toremain on a hydrophobic substrate once it has contacted the substrate.At any given time, there will be more preferentially absorbed surfacemolecules on a surface than there would be non-preferentially absorbedsurface molecules on a comparable surface.

As used herein, “stably associated” with a substrate refers to aninteraction between polymerized, crosslinked surface-modifying moleculesand a substrate that remains intact after one or more washes in anaqueous solution and/or an organic solvent (such as an alcohol), andpreferably, remains intact, after at least about 5, or at least about 10washes. Preferably, a molecule which is “stably associated” with asubstrate is one which remains attached to the substrate after exposureto at least about 90° C., for at least about 2 hours. “Stableassociations” can be monitored by evaluating the wettability (i.e.,hydrophillicity) of a substrate which is coated with difunctionalsurface-modifying molecules according to the invention.

As used herein, “wettable” refers to a membrane which is wetted acrossits entire surface without phobic patches.

As used herein, “a flow-through method” refers to a method where asolution is flowed through a substrate to coat the substrate with thesolution.

Surface-Modifying Molecules

Preferably, the difunctional surface-modifying molecules comprise ahydrophilic portion and a hydrophobic portion. The surface-modifyingmolecules form hydrophobic interactions with the substrate via thehydrophobic portion of the molecules and form substantially no covalentbonds with the surface (e.g., less than about 1%, and preferably, lessthan about 0.1%, or less than about 0.01% of the molecules on thesubstrate are covalently bonded to the substrate). Generally, thesurface modifying molecules also form substantially no ionic bonds withthe surface.

In one particularly preferred aspect, the difunctional moleculecomprises the general formula F-R where F represents the hydrophobicportion of the surface-modifying molecule and R represents thehydrophilic portion.

In one aspect, F is a hydrophobic alkyl, aromatic group, or olefinichydrocarbon group. Preferably, F is selected from the group consistingof a hydrocarbon backbone (straight chained, branched or cyclic) havingat least six carbons and preferably, up to about 50 carbons. In onepreferred embodiment, F comprises an aromatic hydrocarbon molecule, or asubstituted form thereof. Exemplary aromatic hydrocarbon moleculesinclude, but are not limited to, phenols, benzyls, benzoyls, naphthyls;substituted forms thereof; and combinations thereof. In one particularlypreferred embodiment, F is a bisphenol A. Preferably, F does notcomprise hydroxyl, carboxyl or amino groups, i.e., F is not capable ofcovalently bonding to the substrate.

Preferably, R comprises at least two crosslinking active groups.Preferably, at least one of the groups comprises a carbon-carbon doublebond. R may be positively charged, negatively charged, or nonionic,depending on the desired properties of the membrane (for example, amembrane for capturing and immobilizing nucleic acids, preferablycomprises positively charged hydrophilic groups). Suitable R groupsinclude, but are not limited to, acrylates, which may optionally includeone or more alkyl groups, cyclic ring groups containing one of morehetero atoms, and hydrophilic groups, such as hydroxy, ethoxy, carboxyor amino groups.

Suitable hydrophilic groups comprise the general formula[—X_(n1)—Y—CR═CH₂]_(n2) where X is independently selected from the groupincluding, but not limited to, X═(—CH2-CH2-O—);(—CH2-O—);(—CH2-CH(COOH)—); (—CH2-CH(OH)—), Y can include, but is notlimited to ([—CH2-]n3); (—COO—); and n₁ is from about 1-50 while n₂ isfrom about 1-2. n₃ can be from about 1 to about 50.

However, these are only exemplary atoms that might be used, and itshould be obvious to those of skill in the art that others might besubstituted, so long as the hydrophilic nature of R is maintained, andthat such substitutions are encompassed within the scope of theinvention.

A general classification scheme for “hydrophilic” and “hydrophobic”biomaterial surfaces is provided in J. Biol. Mat. Res. 20, pp. ix-xi(1986).

The difunctional surface-modifying molecules according to the inventionare coated on the surface as monomers in solution and then polymerizedusing a free radical initiator, such as a photoinitiator (also free insolution), which adds free radicals to the carbon-carbon double bonds ofthe surface-modifying molecules. Both the surface-modifying molecule andthe photoinitiator molecules are preferentially adsorbed on thesubstrate surface prior to cross-linking. As shown in FIG. 1B, thisenhances the efficiency of the coating process, since surface-modifyingmolecules are more likely to remain on the surface of the substrate thannot, and can interact highly efficiently with photoinitiator moleculeswhich are also preferentially deposited and concentrated on thesubstrate surface.

Polymerization takes place across the crosslinking active groups, assurface-modifying molecules comprising free radicals interact with othersurface-modifying molecules at their crosslinking groups. The result isthe formation of a polymeric network on the surface of the substratewith hydrophilic properties.

Because of the preferential absorption of surface-modifying molecules onthe substrate surface, as well as the preferential absorption ofphotoinitiators on the substrate surface, crosslinking betweenpolymerized molecules is less likely to form the inter-connectingnetworks shown in seen for membranes of the prior art (see, e.g., FIG.1A), and therefore less pore plugging. Preferably, the pore sizes of thecoated membrane are substantially the same as the pore sizes of thehydrophobic substrate.

In one particularly preferred embodiment, the difunctionalsurface-modifying molecules are ethoxylated (30) bisphenol A diacrylateswhich are available from Sartomer (Oaklands Corporate Center, Exton Pa.19341) under catalog number CD9038. While mono-functional hydrophilicacrylate monomers are usually used to hydrophilize a hydrophobicmembrane surface. Only minute amounts of difunctional acrylate monomersare used as cross-linking agent. It is a discovery of the instantinvention that by using difunctional acrylate monomers predominantly, ifnot exclusively, membrane hydrophilization can be achieved using muchless reagent. Reducing the amount of reagents used provides twoadvantages: (1) less pore plugging and (2) reduced down stream washingrequirements. Judicious selection of a difunctional acrylate monomerwith a hydrophobic section within the molecule, promotes thehydrophobic-hydrophobic interaction between the difunctional acrylateand the membrane surface, thereby increasing the efficiency of thehydrophilization process.

Surface-modifying molecules according to the invention may have one ormore of the following properties: resistance to degradation uponexposure to aqueous solutions, such as biological solutions; resistanceto degradation by solvents; biocompatibility (e.g., membrane surfacesshould not induce significant: platelet adhesion, interfere with thenormal clotting mechanism; or cause any significant damage to thecellular elements or soluble components of the blood); and minimal poreclogging. Preferably, composite membranes comprising surface-modifyingmolecules according to the invention are chemically inert.

Hydrophobic Substrates

Suitable hydrophobic porous substrates include, but are not limited to:polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF),polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP),polystyrene, polysulfone, polyethersulfone, Polycarbonates (PC),Polyetheretherketone (PEEK), Polyetherimide (PEI), Polymethylpentene(PMP), Polyphylene Oxide (PPO), Polyphenylene Sulfide (PPS), PolyvinylChloride (PVC), PolyStyrene-Acrylonitrile (SAN), polyolefins (e.g., suchas polyethylene or polypropylene), polyester substrates, (e.g., such asPolyethylene Terephthalate (PET) and Polybutylene Terephthalate (PBT)),copolymers of ethylene and tetrafluroethylene (ETFE), copolymers ofethylene and chlorotrifluroethylene (ECTFE), copolymers of PVDF withchlorotrifluoroethylene (CTFE); and copolymers of TFE, HFP andvinylidene fluoride (VDF). The substrate may comprise fibers that may bewoven or non-woven. In one aspect, the hydrophobic substrate is amultilayer substrate comprising a hydrophobic surface (e.g., thesubstrate may comprise an organic polymer such as PMMA, aliphaticpolyurethane, or a polyolefin copolymer having a fluoropolymer surfacelayer).

The substrate may be in the form of a membrane, a film, a web, a mesh, afabric, a matrix, and more generally is in any form that provides asurface. The particular form or use of the substrate is not intended tobe a limiting aspect of the invention.

Substrates (and the composite membranes formed from these substrates)may be characterized by their hydraulic permeability and sievingcoefficient. In one aspect, a hydrophobic substrate has a hydraulicpermeability for water, at 25° C., of at least about 10.0 ml/m²/hr/cmHg.“Hydraulic permeability” is defined as the volume of a solventtransported through the membrane under the influence of a pressuregradient. In one aspect, hydrophobic substrates according to theinvention have an average pore size of from about 0.01 μm to 10 μm,i.e., are suitable for microfiltration. Membranes also can be providedwhich are suitable for ultrafiltration. Preferably, such membranes havemolecular weight cutoff values of 10 kDa or less, 30 kDa, 50 kDa, 100kDa, or higher and pore sizes less than 0.1 μm. Pores may have uniformsizes on average or may comprise varying sizes.

In one aspect, a hydrophobic substrate according to the invention has aminimum flow rate of 10 ml/min/cm², for a 0.2 μm pore size, under apressure, of 10 psi.

In one preferred aspect, the hydrophobic substrate is a membrane whichis formed as a thin sheet (e.g., approximately 80-150 μm, preferably,about 120 μm) of substantially uniform thickness.

In preferred aspect, a hydrophobic substrate according to the inventioncomprises a PVDF membrane. PVDF membranes are commercially availablewith average pore sizes (i.e., pore diameters) in the range from about0.05 μm to about 10.0 μm. The smallest of these conventional pore sizeswill retain some large viruses and most bacteria. Aconventionally-produced PVDF membrane is disclosed by Mahoney, in U.S.Pat. No. 5,013,339.

Methods of Making Composite Membranes

The invention further provides a method for making a composite porousmembrane comprising a hydrophobic substrate and a hydrophilic surface.In one aspect, a hydrophobic porous membrane is rendered hydrophilic ina continuous process by coating directly in a flow through mode withpreferentially absorbing monomeric difunctional surface-modifyingmolecules, and photoinitiator molecules such as those described above.

The difunctional surface-modifying molecules have integral hydrophobicregions in the molecules which form associations with the substrate,facilitating preferential absorption of the surface-modifying moleculeson the substrate. The hydrophilic portions of the surface-modifyingmolecules are extended away from the substrate, providing a hydrophilicsurface on the substrate while maintaining the underlying porousstructure of the substrate.

In one aspect, a hydrophobic substrate, such as a PVDF membrane, istreated with a reagent bath containing difunctional surface-modifyingmolecules. As used herein, “treated” refers to forcing the solutionthrough the membrane for sufficient periods of time to coat thesubstrate with the difunctional surface-modifying molecule and thephotoinitiator molecules. Preferably, the bath additionally comprises asolvent to facilitate wetting, and to dissolve the difunctionalsurface-modifying molecule and also a mixture of suitable solvents, suchas, for example, a mixture of water with an alcohol, can be used.

More preferably, the photoinitiator comprises of a phobic photoinitiatorcapable of the preferential adsorption on the surface of the substrate.Exemplary photoinitiator molecules of this type include those shown inFIGS. 2A-2O.

Suitable photoinitiators are agents that can initiate radicalcrosslinking. Such agents are known in the art and include, but are notlimited to, 1-[4-(2-Hydroxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one(Ciba Irgacure 2959) and other related Irgacures; benzoin methyl ether;1-hydroxycyclohexylphenyl ketone; and Darocur-related molecules, such asDarocur 1173.

Preferably, exposure to a reagent bath comprising a surface-modifyingmolecule and a photoinitiator is for a period of time from within about0-10 minutes; longer coating periods may be used but are not necessary.

Crosslinking can be triggered by actinic radiation, such as, forexample, by UV light, or ionizing radiation, such as, for example, gammaradiation or X-radiation.

Exposure to actinic radiation can be effected at room temperature for atime usually between about 1 to 120 seconds, and preferably, betweenabout 5 and 60 seconds. Exposure converts the monomeric form of thedifunctional surface-modifying molecules to polymers. Upon completion ofcrosslinking, the photoinitiator and excess monomer, if present, areremoved from the composite porous membrane by rinsing in a suitablesolvent. When the composite porous membrane is dried, it has essentiallythe same porous configuration as the original porous hydrophobicsubstrate.

While passive immersion (i.e., dipping, spraying, etc.) can be used tocoat the substrate and generate porous composite membranes according tothe invention, in one particularly preferred embodiment, an activemethod, such as a flow-through method is used.

Preferably, the substrate is placed in a flow device comprising aprocess chamber which contains the reagent bath. Solution comprising thedifunctional surface-modifying molecules, solvent, and photoinitiator isforced through a hydrophobic substrate. The substrate may be supportedon a porous support during this process or on a frame (e.g., such as therim of the opening of a flask). Solution may be forced through thesubstrate using a pressure differential, e.g., such as by applying avacuum and withdrawing the solution into a waste receptacle. Generally,the flow rate is optimized to achieve satisfactory coating levels (e.g.,such as a maximum amount of hydrophilicity at the membrane surface for aminimum amount of surface modifying molecule). The membrane is thenexposed to UV light to permit the surface-modifying molecules topolymerize and to crosslink. The membrane is then dried and can bestored until ready to use.

It is a discovery of the instant invention that a flow-through method ofcoating can be highly efficient in generating composite membranes. Muchless surface-modifying molecules are used. For example, in comparison topassive immersion methods, for an incubation time of less than 5minutes, less than about 0.5% of difunctional surface-modifying monomercan provide an effective coating, in comparison to levels of about 6-12%required in passive immersion methods.

The porous composite membranes treated according to the invention havegreater liquid flow rates per unit area with equal particle retention ascompared to a membrane treated by other treatment methods of prior art.This means that if a sample of a solution containing particles is passedthrough the membrane of the present invention and an equal volume sampleof the same solution is passed through a membrane treated byconventional methods, both membranes will retain the same amount ofmaterial, but the membrane of the present invention will have a fasterflow rate and process the liquid volume in a shorter time period. Thecomposite porous membranes generated using the flow-through methoddescribed herein, have flow rates that equal those of the originalphobic membrane in contrast to prior art membranes such as the one shownin FIG. 1A.

Preferably, the membranes can be wetted even after drying for at least 2hours at 90° C. Wetting times range from about 0 to less than a minute,preferably, less than 30 seconds, and still more preferably, in lessthan about 15 seconds.

The composite membranes can be used in a variety of applications,including, but not limited to, liquid separation processes such asmicrofiltration, ultrafiltration, dialysis, capture and/orimmobilization of biomolecules (e.g., nucleic acids, proteins,polypeptides, peptides, viruses, cells, and the like); as surfaces forimplantation into the body (e.g., as part of an implantable medicaldevice), or as surfaces incorporated into different devices.Additionally, the membranes can be configured in a variety of formsincluding, but not limited to, flat sheets, hollow fibers or tubes, andcan be any shape.

EXAMPLES

The invention will now be further illustrated with reference to thefollowing examples. It will be appreciated that what follows is by wayof example only and that modifications to detail may be made while stillfalling within the scope of the invention.

Example 1

A hydrophobic PVDF microporous membrane, having an average pore size of0.45 micron and average thickness of 120 microns, was renderedhydrophilic by treating with a reagent bath containing a difunctionalacrylate monomer and a photoinitiator in 50/50 IPA water solventmixture. The difunctional acrylate monomer was ethoxylated (30)bisphenol A diacrylate (Sartomer CD 9038) and the photoinitiator was1-[4-(2-Hydroxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one (CibaIrgacure 2959). Three membrane samples were treated in a flow throughmode while another 3 samples were treated in a passive immersion mode.The treated membranes were UV irradiated, washed and dried. Theconcentration used for the treatments and the wettability of theresulting membranes is listed in the following table. The totaltreatment time was equal to 5 minutes in both cases.

The results demonstrated an instant wetting of the flow-through treatedmembranes versus an uneven wetting, with a lot of phobic spots, in thecase of the passively treated membranes, in spite of the difference ofthe concentration of the monomer used, which was half as much in thecase of the flow through application.

Wettability after drying at 90° C., No composition treatment 18 hours 10.5% CD9038; 1.5% Irgacure 2959; Flow-through wet 50/50 H₂O/IPA 2 0.5%CD9038; 1.5% Irgacure 2959; Flow-through wet 50/50 H₂O/IPA 3 0.5%CD9038; 1.5% Irgacure 2959; Flow-through wet 50/50 H₂O/IPA 4 1% CD9038;1.5% Irgacure 2959; passive spotty 50/50 H₂O/IPA 5 1% CD9038; 1.5%Irgacure 2959; passive spotty 50/50 H₂O/IPA 6 1% CD9038; 1.5% Irgacure2959; passive spotty 50/50 H₂O/IPA

Example 2

A polyvinylidene difluoride (PVDF) microporous membrane, having anaverage pore size of 0.2 micron and average thickness of 120 microns, istreated to produce a hydrophilic surface.

One set of 2 samples was treated in a flow-through mode with the reagentbath containing 1% of ethoxylated (30) bisphenol A diacrylate (SartomerCD 9038) and 0.5%1-[4-(2-Hydroxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one (CibaIrgacure 2959).

A second set of samples was treated in a flow-through mode with thereagent bath containing 1% of polyethylene glycol (400) diacrylate(Sartomer SR 344) and 0.5%1-[4-(2-Hydroxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one (CibaIrgacure 2959).

Ethoxylated (30) bisphenol A diacrylate (Sartomer CD 9038) has ahydrophobic anchor and is capable of preferential adsorption on phobicsubstrates, while polyethylene glycol (400) diacrylate (Sartomer SR 344)is a totally hydrophilic molecule, and is not capable of a significant,preferential adsorption on the phobic substrates.

The membrane samples were pre-wetted with IPA and washed with watermixture before the treatment. The treated membranes were UV irradiated,washed and dried.

The results listed in the following table showed that monomers capableof the preferential adsorption can be used at concentration levels whereother monomers do not work.

Wettability after drying at No composition treatment 90° C., 18 h 1 1%CD9038; 0.5% Irgacure 2959 Flow-through wet 2 1% CD9038; 0.5% Irgacure2959; Flow-through wet 3 1% SR344; 0.5% Irgacure 2959; Flow-throughphobic 4 1% SR344; 0.5% Irgacure 2959; Flow-through phobic

Example 3

A PVDF microporous membrane, having an average pore size of 0.2 micronsand average thickness of 120 microns, was treated to produce ahydrophilic surface. Two samples were treated in a flow through modewith the reagent bath containing 2% of ethoxylated (30) bisphenol Adiacrylate (Sartomer CD 9038) and 0.5% Ciba Irgacure 2959 in 70/30water/IPA solvent. The third sample was used as an untreated control.The treated membranes were UV irradiated, washed and dried.

The results after washing and drying of the membranes demonstrated aninstant wetting of the flow-through treated membranes. The performancecharacteristics revealed no differences between treated and non-treatedmembranes.

Wettability after drying IPA at 90° C., bubble number compositiontreatment 18 hours point Control NA NA phobic 24 11 2% cd9038; 0.5%Irgacure Flow- wet 24 2959; 70/30 H₂O/IPA through 12 2% cd9038; 0.5%Irgacure Flow- wet 24 2959; 70/30 H₂O/IPA through

Example 4

A polyvinylidene difluoride (PVDF) microporous membrane, having anaverage pore size of 0.2 micron and average thickness of 120 microns wastreated to produce a hydrophilic surface.

One set of 6 samples was treated in a flow-through mode with the reagentbath containing 2% ethoxylated (30) bisphenol A diacrylate (Sartomer CD9038) and 0.125% 1-Hydroxy-cyclohexyl-phenyl-ketone (Ciba Irgacure 184)in an 85/15 water/IPA solvent.

A second set of samples was treated in a flow-through mode with thereagent bath containing 2% of ethoxylated (30) bisphenol A diacrylate(Sartomer CD 9038) and 0.25% 1-Hydroxy-cyclohexyl-phenyl-ketone (CibaIrgacure 184) in 80/20 water/IPA solvent.

The membrane samples were pre-wetted with IPA and washed with awater/solvent mixture before the treatment. The treated membrane was UVirradiated, washed and dried.

The results listed in the following table show that it is possible toproduce a hydrophilic membrane capable of withstanding extensive drying,and that it is possible to optimize the hydrophilization process bychanging the compositions of the different reagent baths.

Wettability Wettability after drying after drying at 90° C., 2 at 90°C., No Composition Treatment hours 18 hours 1 2% cd9038; Flow-throughinstant 15.7 ± 11.8 sec 0.125% Irgacure 184; 85/15 H₂O/IPA 2 2% cd9038;Flow-through instant 1.5 ± 1.6 sec 0.25% Irgacure 184; 80/20 H₂O/IPA

Example 5

Three rolls of PVDF microporous membrane (600 ft each), having anaverage pore size of 0.45 micron and average thickness of 120 micronswere treated to produce a hydrophilic surface. All 3 rolls were treatedin a flow-through mode with the reagent bath containing 2% ethoxylated(30) bisphenol A diacrylate (Sartomer CD 9038) and 0.25%1-Hydroxy-cyclohexyl-phenyl-ketone (Ciba Irgacure 184) in an 80/20water/IPA solvent.

Treated membranes were UV-irradiated, washed and dried.

The results listed in the following table showed that it is possible toproduce a hydrophilic membrane capable of withstanding both theextensive drying, and autoclaving, using very low concentrations of thechemicals. Results of the treatment were absolutely consistentthroughout the whole length of the treated membrane.

WETTABILITY 2 h, Sample Intact 90° C. 18 h, 90° C. 3 Autoclave CyclesRoll 1  0 ft instant Instant instant instant 200 ft instant Instantinstant instant 400 ft instant Instant instant instant 600 ft instantInstant instant instant Roll 2  0 ft instant Instant instant instant 200ft instant Instant instant instant 400 ft instant Instant instantinstant 600 ft instant Instant instant instant Roll 3  0 ft instantInstant instant instant 200 ft instant Instant instant instant 400 ftinstant Instant instant instant 600 ft instant Instant instant instant

Example 6

A supported polytetrafluoroethylene microporous membrane (PTFE), havingan average pore size of 1 micron, and non-woven polypropylene (PP),having an average pore size of 0.5 micron, were treated to produce ahydrophilic surface.

Samples were treated in a flow-through mode with the reagent bathcontaining 2% of ethoxylated (30) bisphenol A diacrylate (Sartomer CD9038) and 0.25% 1-Hydroxy-cyclohexyl-phenyl-ketone (Ciba Irgacure 184)in 80/20 water/IPA solvent.

The membrane samples were pre-wetted with IPA and washed with waterbefore treatment. The treated membranes were UV irradiated, washed anddried.

The results listed in the following table showed that it is possible toproduce a hydrophilic surface capable of withstanding extensive drying.

Wettability Wettability after after drying at drying at 90° C., 90° C.,No Composition Treatment 2 hours 18 hours PTFE 2% cd9038; Flow-throughinstant instant 0.25% Irgacure 184; 80/20 H₂O/IPA PP 2% cd9038;Flow-through instant instant 0.25% Irgacure 184; 80/20 H₂O/IPA

Example 7

A polyvinylidene difluoride (PVDF) microporous membrane, having anaverage pore size of 0.45 micron and average thickness of 120 microns,was treated to produce a hydrophilic surface.

One set of 8 samples was treated in a flow-through mode with the reagentbath containing 2% of ethoxylated (30) bisphenol A diacrylate (SartomerCD 9038) and 0.25% 1-Hydroxy-cyclohexyl-phenyl-ketone (Ciba Irgacure184) in 80/20 water/IPA solvent.

The membrane samples were pre-wetted with IPA and washed with a watermixture before the treatment. The treated membranes were UV irradiated,washed and dried.

The second set of 8 samples was left untreated (phobic).

Wettability, water flow rate and IPA bubble point were measured for allthe samples. The results listed in the following table showed that it ispossible to produce a hydrophilic membrane capable of withstanding theextensive drying with virtually no change in the flow throughefficiency.

Wettability Water IPA bubble after drying at flow rate point NoComposition 90° C., 18 hours cc/min/cm² psi 1 2% cd9038; 0.25% instant52.78 ± 1.79 11.38 ± 0.21 Irgacure 184; 80/20 H₂O/IPA 2 None phobic51.14 ± 3.78 11.15 ± 0.18

All patent and non-patent publications cited in this specification areindicative of the level of skill of those skilled in the art to whichthis invention pertains. All these publications and patent applicationsare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated as being incorporated by reference herein.

Those skilled in the art will recognize, or be able to ascertain, usingno more than routine experimentation, numerous equivalents to thespecific substances and procedures described herein. Such equivalentsare considered to be within the scope of this invention, and are coveredby the following claims.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A composite porous membrane comprising: a hydrophobic substratehaving an average pore size ranging from about 0.01 μm to about 10 μmcoated with difunctional surface-modifying molecules; each difunctionalsurface-modifying molecule comprising a hydrophobic portionpreferentially associated with the substrate and a hydrophilic portionand having an active group containing a carbon-carbon double bond; thedifunctional surface-modifying molecules consisting of a difunctionalacrylate monomer; wherein the difunctional acrylate monomer comprisesgreater than about 90% of the molecules associated with the membrane;wherein the substrate is coated by flowing a reagent solution throughthe substrate to coat the substrate surface and inner surfaces of thepores, the reagent solution consisting of the difunctionalsurface-modifying molecules, a solvent, and a photoinitiator, whereinthe reagent is capable of flowing through the substrate; and wherein thesurface-modifying molecules are crosslinked to form a crosslinkedhydrophilic polymeric network at the substrate surface and innersurfaces of the pores of the membrane, and wherein the pore size of thecoated membrane is substantially the same as the pore size of the porousmembrane before coating.
 2. The membrane according to claim 1, whereinthe hydrophilic portion of the surface-modifying molecules comprises atleast two crosslinking active groups.
 3. The membrane according to claim2, wherein the crosslinking active group comprises a carbon-carbondouble bond.
 4. The membrane according to claim 1, wherein 100% ofmolecules associated with the substrate comprise difunctionalsurface-modifying molecules.
 5. The membrane according to claim 1,wherein the hydrophobic portion is a hydrophobic alkyl, aromatic group,or olefinic hydrocarbon group.
 6. The membrane according to claim 1,wherein the hydrophobic portion comprises an aromatic hydrocarbonmolecule.
 7. The membrane according to claim 6, wherein the aromatichydrocarbon comprises a bisphenol A group.
 8. The membrane according toclaim 1, wherein the hydrophobic portion does not form covalent bondswith the surface.
 9. The membrane according to claim 1, wherein thehydrophilic portion is positively charged.
 10. The membrane according toclaim 1, wherein the hydrophilic portion is negatively charged.
 11. Themembrane according to claim 1, wherein the hydrophilic portion comprisesa neutral charge.
 12. The membrane according to claim 1, wherein thehydrophilic portion comprises the general formula[—X_(n1)—Y—CR═CH₂]_(n2) where X is independently selected from the groupconsisting of (—CH2-CH2-O—); (—CH2-O—); (—CH2-CH(COOH)—);(—CH2-CH(OH)—); Y is selected from the group consisting of([—CH2-]_(n3)); (—COO); n₁ is from about 1-50; n₂ is from about 1-2; andn₃ can be from about 1 to about
 50. 13. The membrane according to claim1, wherein the difunctional surface modifying molecules are polymerizedon the substrate surface after being preferentially adsorbed with thesubstrate surface.
 14. The membrane according to claim 1, wherein thedifunctional surface molecules comprise ethoxylated (30) bisphenol Adiacrylates.
 15. The membrane according to claim 1, wherein thedifunctional-surface molecules are polymerized using a photoinitiator,and wherein the photoinitiator is preferentially adsorbed by thesubstrate surface.
 16. The membrane according to claim 1, wherein thedifunctional-surface molecules are polymerized using a photoinitiatorthat comprises a substantially hydrophobic molecule.
 17. The membraneaccording to claim 1, wherein the difunctional-surface molecules arepolymerized using a photoinitiator selected from the group consisting of1-hydroxy-cyclohexyl-phenyl-ketone;2-benzyl-2-dimethylamino-1-(4-morpholinophenyl1)-butanone-1; 50%1-hydroxy-cyclohexyl-phenyl-ketone and 50% benzophenone; 25%bis(2,6-dimethoxybenzoyl)-2,4,4-trimethyl pentylphosphineoxide and 75%2-hydroxy-2-methyl-1-phenyl-propan-1-one;2,2-dimethoxy-1,2-diphenylethan-1-one;bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide; 80%2-hydroxy-2-methyl-1-phenyl-propan -1-one and 20%1-hydroxy-cyclohexyl-phenyl-ketone; 25%bis(2,6-dimethoxybenzoyl)-2,4,4-trimethyl-pentylphosphineoxide and 75%1-hydroxy-cyclohexyl-phenyl-ketone;2-hydroxy-2-methyl-1-phenyl-propan-1-one; benzophenone; 50%2,4,6-trimethylbenzoyl-diphenyl-phosphineoxide and 50%2-hydroxy-2-methyl-1-phenyl-propan-1-one;bis(□5-2,4-cyclopentadien-1-yl)-bis(2,6-dicluoro-3-(1H-pyrrol-1-yl)-phenyl)titanium;2-methyl-1[4-(methylthio)phenyl]-2-morpholinopropan-1-one; 30%2-benxyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 and 70%2,2-dimethoxy-1,2-diphenylethan-1-one; and1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one. 18.The membrane according to claim 1, wherein the hydrophobic substratecomprises polyvinylidene fluoride.
 19. The membrane according to claim1, wherein the membrane is wettable within less than about 30 secondsafter drying upon contacting with an aqueous solution.
 20. The membraneaccording to claim 1, wherein the membrane is autoclavable.
 21. Themembrane according to claim 1, wherein the hydrophobic portion iscapable of significant association with the substrate.
 22. The membraneaccording to claim 1, wherein the coating is provided on the hydrophobicsubstrate by exposure to a reagent solution comprising less than 1%difunctional surface-modifying molecule.
 23. The membrane according toclaim 22, wherein the reagent solution comprises less than about 0.5%difunctional surface-modifying molecule.
 24. The membrane according toclaim 22, wherein the reagent solution comprises less than about 0.25%difunctional surface-modifying molecule.
 25. The membrane according toclaim 1, wherein the flow rate through the pores of the coated membraneis substantially the same as the flow rate through the pores of thenon-coated membrane.
 26. The membrane according to claim 25, wherein theflow rate through the pores of the coated membrane is at least about 93%of the flow rate through the pores of the non-coated membrane.
 27. Themembrane according to claim 25, wherein the flow rate through the poresof the coated membrane is at least about 96% of the flow rate throughthe pores of the non-coated membrane.
 28. The membrane of claim 1wherein the hydrophobic substrate has an average pore size of about 0.2μm.
 29. The membrane of claim 1 wherein the hydrophobic substrate has anaverage pore size of about 0.45 μm.